1. Field of the Invention
The invention relates to processes for preparing filterable, deodorized, highly concentrated, aqueous dispersions of pressure-sensitive adhesive, based on copolymers of esters of acrylic and/or methacrylic acid, to products thus prepared and to their use.
2. Description of the Background
Polymer dispersions which are used in the adhesives industry for producing articles having contact adhesion are required to meet stringent quality criteria. Thus it must be possible to reproduce their properties within relatively narrow tolerances in order to enable them to be processed further without problems. In general, the processing of aqueous dispersions is more economic and more environment-friendly than that of systems which include organic solvents, since it does away with the need for complex equipment to recover the organic solvents. Highly concentrated aqueous dispersions of pressure-sensitive adhesive go a step further in terms of economy and environmental compatibility in that less water has to be evaporated in the course of their processing and, consequently, less energy is used than in the processing of aqueous dispersions having a lower polymer content. The transportation of highly concentrated dispersions, moreover, is more efficient than that of dispersions of lower concentration. For the processing of the aqueous polymer dispersions to form everyday products, the dispersions must be substantially free from volatile components. Volatile components are, in particular, unreactive monomers and saturated constituents in the monomers used, it being of course impossible to remove the latter by the polymerization reaction. To remove the volatile components, the polymer dispersions are treated--subsequent to their preparation--with steam (deodorization). For deodorization the polymer dispersions must be of sufficient stability, since otherwise the deodorization procedure is accompanied by the agglomeration of material and the formation of coagulum, reducing the solids content of the dispersions. This coagulation occurs with particular readiness in the case of highly concentrated polymer dispersions. Dispersions containing coagulum are not easy to filter, and before being used further as highly concentrated pressure-sensitive adhesive dispersions they require particularly laborious techniques for the removal of the coagulum.
There has been no lack of efforts to prepare highly concentrated aqueous polymer dispersions with solids contents or polymer contents of more than 65% by weight. For instance, EP-A 0 065 253 describes a process for preparing highly concentrated aqueous polyacrylate dispersions by the technique known as monomer emulsion feed polymerization, the resulting polymer dispersions being used for dressing leather. It is indicated that filterable polymer dispersions are obtained only when use is also made of a specific emulsifier, a dialkyl sulfosuccinate having 8 to 18 C atoms in the alkyls. However, there is great fluctuation in the properties of the resulting highly concentrated polymer dispersions, such as in their viscosities.
EP-B 0 037 923 likewise describes a process for preparing highly concentrated aqueous polyacrylate dispersions by the monomer emulsion feed polymerization technique, although the reproduction of the resulting dispersions does not meet the current quality requirements on account of the fact that marked fluctuations in properties occur here as well. Moreover, the highly concentrated aqueous polymer dispersions obtained are of only limited stability on deodorization. Furthermore, the process involves undesirable restrictions in terms of selection of monomers, content of minimum amounts of inhibitor, and the composition of the aqueous phase of the initial charge to the polymerization reactor, which are disadvantageous.